The present application generally pertains to solar concentrators and more particularly to transparent solar concentrators for integrated solar windows.
Integrating solar-harvesting systems into the built environment is a transformative route to capture large areas of solar energy, lower effective solar cell installation costs, and improve building efficiency. Widespread adoption of solar-harvesting systems in a building envelope, however, is severely hampered by difficulties associated with mounting traditional solar modules on and around buildings due to cost, architectural impedance, and mostly importantly, aesthetics.
The concept of luminescent solar concentrators (“LSCs”) is well known, and with recent advances in phosphorescent and fluorescent luminophore efficiencies, LSC system efficiencies have increased to 7.1%. Although optical funneling of light limits the overall system conversion efficiency to less than ten percent (without LSC stacking), it can dramatically reduce the area of expensive solar cells needed, driving down the overall installed cost and increasing the ratio of electricity generation to solar cell surface area. Because of the high cost of glass and real-estate that factor into the module and the balance of systems costs, respectively, such LSCs have rarely been adopted in solar-farm practice despite the increasing performance and potential for low module costs. Furthermore, there has been demonstrated interest in utilizing LSCs as architectural windows. To date, however, these systems have been limited to absorption and emission (glow) in the visible part of spectrum, hindering widespread adoption of such devices in windows. In general, the purpose of windows is to provide natural lighting with a view; that is, most people prefer not to work behind strongly colored glass. A high level of untinted-transparency is therefore desirable for ubiquitous adoption.
The performance of LSCs can be understood by the component efficiencies: luminophore photoluminescence efficiency (quantum yield), solar spectrum absorption efficiency, waveguide (trapping) efficiency, solar cell efficiency, and transport (re-absorption) efficiency. The highest performance LSCs utilize phosphorescent organic molecules or blends of multiple fluorophores (such as quantum dots or organic dyes) that act to reduce reabsorption (Stokes shift) losses and enhance overall absorption efficiencies across the spectrum. The highest efficiencies reported (6-7%) have been for relatively small plates (<0.1 m2), since larger LSCs sizes suffer substantial reabsorption losses that limit efficiencies to <5%.
It has long been recognized that LSCs are most limited by reabsorption losses, particularly for larger plate sizes. Indeed, much of the research with LSCs has focused on the reduction of these reabsorption losses through increasing Stokes shifts with organic phosphors, multiple dye optimization to artificially increase the Stokes-shift or resonance shifting, applicable only to neat-film dye layers less than several microns thick.
Previous efforts to construct transparent solar-harvesting architectures have focused on: (1) semi-transparent thin-film photovoltaics that typically have severe tinting or limited transmission or have an inherent tradeoff between efficiency and transparency, (2) LSCs incorporating colored chromophores that absorb or emit in the visible, or (3) optical systems using wavelength dependent optics that collect direct light only and require solar tracking. All of these approaches are severely limited in their potential for window applications due to aesthetic properties, bulkiness, or considerably limited transparency. These approaches suffer from an inherent tradeoff between power conversion efficiency (“PCE”) and visible transparency (“VT”), since both parameters cannot be simultaneously optimized in conventional devices. Architectural adoption is impeded further with typical organic photovoltaics (PVs) that have peaked absorption within the visible spectrum, resulting in poor color rendering index (“CRI”), high colored tinting and poor natural lighting quality. In contrast, it would be desirable to obtain visibly transparent, UV/NIR-selective solar concentrators to avoid aesthetic tradeoffs (low VT or CRI) that hinder architectural adoption and provide a clear route to large area scaling.
FIG. 1 shows an illustration of a cross section of a luminescent solar concentrator (LSC) 100 in a traditional configuration. The SC 100 comprises a first inorganic solar cell 105 and a second inorganic solar cell or a reflective film 110, a substrate 115, and a waveguide redirecting material 120. The waveguide redirecting material 120 can be an NIR luminescent dye or scattering particles. Long bracket 125 represents incoming solar flux, short bracket 130 represents NIR light, and block arrow 135 represents visible light. Arrows 140 represent waveguided NIR light. The factors that negatively affect the efficiency of the LSC 100 include, poor luminophore photoluminescence efficiency (quantum yield), poor solar spectrum absorption efficiency, poor waveguide (trapping) efficiency, poor solar cell efficiency, and poor transport (re-absorption) efficiency.
Various conventional devices employ a luminescent solar collector having luminescent or scattering agents dispersed throughout. Exemplary U.S. Patent Nos. include: U.S. Pat. No. 4,155,371 entitled “Luminescent Solar Collector” which issued to Wohlmut et al. on May 22, 1979; U.S. Pat. No. 4,159,212 entitled “Luminescent Solar Collector” which issued to Yerkes on Jun. 26, 1979; U.S. Pat. No. 4,357,486 entitled “Luminescent Solar Collector” which issued to Blieden et al. on Nov. 2, 1982; 2009/0027872 entitled “Luminescent Object Comprising Aligned Polymers having a Specific Pretilt Angle” which published to Debije et al. on Jan. 29, 2009; and 2010/0288352 entitled “Integrated Solar Cell Nanoarray Layers and Light Concentrating Device” which published to Ji et al. on Nov. 18, 2010. All of these references are incorporated by reference herein.